This invention relates generally to cryogenic air separation and, more particularly, to the provision of refrigeration for the cryogenic air separation by the turboexpansion of feed air.
One important aspect in the cryogenic rectification of feed air to produce one or more products such as oxygen and nitrogen, is the provision of refrigeration to the process to drive the rectification. One method for providing such refrigeration is the turboexpansion of a compressed gaseous process stream to generate refrigeration which is then provided into the cryogenic air separation plant. Often the turboexpanded process stream is a feed air stream and the refrigeration is passed into the cryogenic air separation plant for the rectification with the turboexpanded feed air.
The turboexpansion of feed air to generate refrigeration for cryogenic air separation is energy intensive. Any improvement to such turboexpansion operation would be highly desirable.
Accordingly, it is an object of this invention to provide an improved method for carrying out cryogenic air separation wherein refrigeration is provided by the turboexpansion of a feed air stream with reduced power requirements over conventional systems.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for producing at least one product by the cryogenic rectification of feed air comprising:
(A) partially condensing a flow of feed air to produce a first two-phase flow of feed air having a liquid phase portion which is not more than 99 percent of said first two-phase flow of feed air;
(B) passing said first two-phase flow of feed air to a turboexpander, and turboexpanding the said first two-phase flow of feed air in the turboexpander to produce a second two-phase flow of feed air having a liquid phase portion which is less than the liquid phase portion of the first two-phase flow of feed air;
(C) passing the second two-phase flow of feed air to a cryogenic air separation plant comprising at least one column; and
(D) separating the feed air by cryogenic rectification in the cryogenic air separation plant to produce at least one product.
As used herein, the term xe2x80x9ctwo-phase flowxe2x80x9d means a fluid having both a liquid phase and a vapor phase.
As used herein, the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure fluid through a turbine to reduce the pressure and the temperature of the fluid thereby generating refrigeration.
As used herein, the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements which may be structured packing and/or random packing elements. For a further discussion of distillation columns, see the Chemical Engineers"" Handbook fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. The term, double column, is used to mean a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is adiabatic and can include integral or differential contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out, at least in part, at temperatures at or below 150 degrees Kelvin (K).
As used herein, the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term xe2x80x9ccryogenic air separation plantxe2x80x9d means the column or columns wherein feed air is separated by cryogenic rectification, as well as interconnecting piping, valves, heat exchangers and the like.
As used herein, the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d of a column means those portions respectively above and below the midpoint of the column.
As used herein, the term xe2x80x9cproduct oxygenxe2x80x9d means a fluid having an oxygen concentration equal to or greater than 80 mole percent.
As used herein, the term xe2x80x9cproduct nitrogenxe2x80x9d means a fluid having a nitrogen concentration equal to or greater than 97 mole percent.
As used herein, the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily nitrogen and oxygen, such as ambient air.